The present invention relates generally to amplifiers and more specifically to all forms of audio amplifiers and guitar amplifiers. It further relates to a distortion synthesizer which replicates the sounds produced by overdriven vacuum tube amplifiers.
Since semiconductor devices have become viable components for amplifiers, there has been a debate upon the virtues of semiconductor or solid state amplifiers versus the vacuum tube amplifiers. Some believe that tube amplifiers work better because vacuum tubes are more natural amplifiers than semiconductor devices. Some think that semiconductor amplifiers produce a sound that has no warmth; they are too clear until the semiconductor devices saturate, then they are too noisy. Tube amplifiers seem not to give up when overdriven, they seem to try to reach the impossible.
The vacuum tube amplifiers, however, do have some limitations. For some people, the limitation is simply the warmup time and the fragility of vacuum tubes. For guitarists the problems are more serious. A powerful amplifier does not sound right when operating at low levels such as those needed to fill a small room.
The prior art is filled with various attempts to satisfy the guitarists need for the tube amplifier sound with the more reliable semiconductor devices or just to enhance the sound from vacuum tube amplifiers. Moog, in U.S. Pat. No. 4,180,707, simulates the overdriven amplifier with a compressor and a clipper that can produce even harmonics as well as odd harmonics to produce the guitar sound at preamplifier power levels. Claret, in U.S. Pat. No. 4,286,492, modifies the operating point of the amplifier to clip at lower power. Woods, in U.S. Pat. No. 3,860,876, heavily modifies the frequency response of a distorted input. Smith, in U.S. Pat. No. 4,211,893, selectively adds gain in a preamplifier stage to get distortion. Sondermeyer, in U.S. Pat. No. 4,405,832, switchable forces odd harmonic distortion and, in U.S. Pat. No. 4,439,742, creates a soft crossover distortion, also an odd harmonic phenomenon. Scholz, in U.S. Pat. No. 4,143,245, creates distortion at any sound level by operating an amplifier at maximum levels with resistive loads and driving the speaker with only a portion of the amplifier output.
In another vein, Todokoro, in U.S. Pat. No. 4,000,474, simulates a triode tube amplifier with junction field effect transistors.
The prior art is also filled with many examples of design built by many manufacturers of guitar amplifiers. Technically, the guitar amplifier is a poor example of vacuum tube amplifier design. Certainly, the guitar amplifier of today does not have the frequency response nor the clarity of the high fidelity amplifier of yesterday. However, the reason is not technical but lies in the art. The sound of inexpensive, overdriven amplifiers became part of the art.
Thus, to simulate the vacuum tube amplifier, one must fully appreciate the nature of the components of said amplifier. One of the key components of a vacuum tube amplifier is the output transformer. The transfer passes a relatively narrow band of frequencies in the middle of the audio spectrum. The feedback in the power stage of the amplifier broadens the transformer response so that the amplifier operates effectively over a wider range of frequencies. However, when the amplifier tubes are saturated, they cannot perform the feedback function and the response narrows to the transformer response.
Of course, another key element of the vacuum tube amplifier is the tube itself. The various stages of a vacuum tube amplifier are usually Coupled with capacitors that carry the signal from the output of one stage to the input of the next while blocking the constant or DC voltage of the output from the input. When the input of a stage is driven so that the grid of the vacuum tube becomes more positive than its cathode, then significant currents will flow in the grid circuit. Some of the grid current charges the coupling capacitor and thereby alters the operating point of the vacuum to amplify more asymmetrically than it usually does. When this asymmetrical waveform is amplified and clipped symmetrically by a push-pull output stage, as usually found in powerful amplifiers, it produces even as well as odd distortion harmonics. It is the even harmonics that seem to be more musical than the harsher odd harmonics.
Another feature of vacuum tubes and their circuitry is that vacuum tubes permit the plate voltage to become considerably more negative than the cathode. Transistors, however, saturate and then blow out their emitter junction The solution, simply put, is to incorporate a diode to bypass the current from the collector or drain to the emitter or source. This extra current path is not found in tube amplifiers and interferes with the ability of the transformer and speaker inductances to flyback.
Thus, the primary object of the invention is to provide a semiconductor amplifier which simulates the distortion of a vacuum tube amplifier. The most important feature of this simulation is the bandwidth narrowing at elevated power levels.
Another object of the invention is to simulate the effects of grid current flowing This produces even harmonic distortion which is a more pleasant and musical distortion than one made of solely odd harmonics. Further, the grid current effects in a capacitively coupled circuit produces the desirable attach on a note.
A further object of the invention is a guitar amplifier effects preamplifier which may be elegantly professional or may be a simple effects pedal. This is quite possible from the teachings herein because the tube simulation may be done either at low or at high power levels.
A further object of the invention is to provide a semiconductor power amplifier which permits the speaker inductance to flyback to voltages greater than the supply voltage so that the speaker current can reverse as fast as possible.
A still further objection of the invention is to achieve the general improvement of guitar amplifiers of all types to provide the high power distortion effect at all power levels.
An even further object of the invention is to provide a general use power amplifier which graciously handles excessive inputs.
These and other objects of the invention are attained by providing a distortion synthesizer having a transformless filter of a defined bandwidth and providing a circuitry for narrowing the bandwidth of the filter as a function of the amplitude of the input signal. The filter generally includes a bandpass filter and an input amplifier. The narrowing circuit includes a limiter connected between the output of the amplifier and the input of the filter and a feedback connecting the output of the filter to the amplifier. The limiter is actuated at a given level of input signal to limit the amplitude to reduce the feedback capability and consequently narrow the frequency response. The frequency response of the filters are adjustable.
A bias shifter may be provided between the input terminal and the input to the amplifier for asymmetrically transmitting the input signals. The bias shifter in combination with the limiter introduces even harmonics. The base shifter is also actuated above a given input signal. A capacitor and a symmetrical resistance of the bias shifter have a time constant sufficient to prolong the bias shifting to allow note attack time before power limiting.
In an alternate embodiment, the filter may include a high-pass and low-pass filter with variable resistances and the narrowing circuit would sense the amplitude of the input signal and vary the values of the resistance and narrow the bandwidth. This is a feed forward instead of a feedback embodiment.
In using semiconductor output stages, it is desirable to bring the flyback into the inaudible range. This includes placing commutating diodes in parallel with the output transistor and providing zener diodes in series with the conduction path of the output push-pull transistors. The zener diodes breakdown in reverse bias and allow the output terminal to exceed the rail voltage to decrease the flyback dissipation time.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.